The present invention relates to a high performance three-way catalyst (TWC) containing an inner and an outer layer on an inert carrier body. The layers comprise noble metals from the platinum group deposited on support materials.
Three-way catalysts are primarily used to convert the pollutants carbon monoxide (CO), hydrocarbons (HC) and nitrogen oxides (NO.sub.x) contained in the exhaust gas of internal combustion engines into harmless substances. Known three-way catalysts with good activity and durability utilize one or more catalytic components from the platinum group metals such as platinum, palladium, rhodium and iridium deposited on a high surface area, refractory oxide support, e.g., a high surface area alumina. The support is usually carried in the form of a thin layer or coating on a suitable carrier or substrate such as a monolithic carrier comprising a refractory ceramic or metal honeycomb structure.
The ever increasing demand for improved catalyst activity and life has led to complex catalyst designs comprising multiple catalyst layers on the carrier structures, each of the layers containing selected support materials and catalytic components as well as so called promoters, stabilizers and oxygen storage compounds.
U.S. Pat. No. 5,063,192 describes a three-way catalyst with improved resistance to thermal stresses which consists of a first and a second catalyst layer. The first layer is directly coated onto the surface of a monolithic honeycomb carrier and comprises active alumina and deposited thereon catalytic components comprising platinum and/or rhodium and at least one compound from zirconia, lanthana or barium oxide. The second layer is coated on top of the first layer and comprises active alumina, ceria and a catalytic component which comprises palladium. The oxides of zirconium, lanthanum and/or barium prevent the particles of active alumina from sintering due to high exhaust gas temperatures and thereby improve thermal resistance of the three-way catalyst.
U.S. Pat. No. 5,677,258 describes a three-way catalyst containing barium oxide with improved resistance against poisoning with sulphur and water. The catalyst consists of two layers on a honeycomb carrier. The lower catalyst layer is located directly on the carrier and comprises at least barium or lanthanum. The upper layer comprises a water adsorbing component. The catalyst further comprises a catalytically active metal which is located at least in the lower or upper layer. In a special embodiment the lower layer further comprises palladium and active alumina and the upper layer further comprises platinum and rhodium.
U.S. Pat. No. 5,057,483 discloses a three-way catalyst comprising two discrete layers on a monolithic carrier. The first, lower layer comprises a first activated alumina support, a catalytically effective amount of a first platinum catalytic component dispersed on the first alumina support, and a catalytically effective amount of bulk ceria. The second or outer layer comprises a co-formed rare earth oxide-zirconia support, a catalytically effective amount of a first rhodium catalytic component dispersed on the co-formed rare earth oxide-zirconia support, a second activated alumina support, and a catalytically effective amount of a second platinum catalytic component dispersed on the second alumina support.
PCT-Publication WO 95/35152 discloses another three-way catalyst, consisting of two layers, which is thermally stable up to 900.degree. C. or more. The first layer comprises a first support; at least one first palladium component, optionally a first platinum group component; optionally at least one first stabilizer; optionally at least one first rare earth metal component and optionally a zirconium compound. The second layer comprises a second support; a second platinum component; a rhodium component; a second oxygen storage composition comprising a diluted second oxygen storage component; and optionally a zirconium component.
German publication DE 197 26 322 A1 describes a three-way catalyst which exhibits improved activity and thermal stability and which consists of two layers on an inert carrier. The first or lower layer comprises several particulate materials and one or more highly dispersed alkaline earth metal oxides and at least one platinum group metal which exhibits an intimate contact with all components of the first layer. The particulate materials of the first layer comprise at least one particulate oxygen storage material and at least one further particulate component. The second layer comprises again several particulate materials and at least one platinum group metal. The particulate materials of the second layer comprise at least a particulate oxygen storage material and a further particulate component. The platinum group metals of the second layer are deposited selectively on the particulate materials of the second layer. Preferably the platinum group metal in the first layer is palladium and the platinum group metals of the second layer are platinum and rhodium.
This latter three-way catalyst exhibits excellent catalytic activity especially during the cold start phase of modern internal combustion engines which are operated with lean air/fuel mixtures during cold start to increase the exhaust gas temperature as fast as possible. The excellent behavior of the catalyst is essentially due to the use of palladium which under lean exhaust gas conditions yields lower light off temperatures than platinum. Despite its excellent performance this catalyst faces the problem that there has developed a shortage in palladium supply during the last few years resulting in rising prices and an uncertain supply situation.
A further problem with existing three-way catalysts is the fact that they suffer under fuel-cut aging. The term fuel-cut aging describes catalyst performance degradation due to fuel-cut after high load operation of the internal combustion engine. Such a situation occurs frequently during fast driving phases when abrupt deceleration is required. During fast driving phases the engine is operated at air/fuel ratios slightly below the stoichiometric value. The exhaust gases may reach temperatures well above 800.degree. C. resulting in even higher catalyst temperatures due to the exothermic conversion reactions at the catalyst. In case of abrupt deceleration modern motor electronics completely stop fuel supply to the engine with the result that the normalized air/fuel ratio (also called lambda value .lambda.) of the exhaust gas jumps from rich to lean values.
These large excursions of the normalized air/fuel ratio from rich to lean values at high catalyst temperatures degrade catalytic activity. Catalytic activity can at least partly be recovered by prolonged operation under stoichiometric exhaust gas conditions. The faster catalytic activity is regained after fuel-cut aging, the better is the overall catalyst performance. Speeding up recovery of catalytic activity after fuel-cut aging is therefore mandatory for modern three-way catalysts.
An object of the present invention is to develop a three-way catalyst based on platinum and rhodium which exhibits a similar catalytic performance as known palladium/rhodium catalysts and which is commercially competitive to the latter. Further, after high temperature aging under lean exhaust gas conditions, the catalyst should recover its full three-way efficiency quickly. The catalyst should also exhibit an improved nitrogen oxide conversion to reduce the ozone forming potential of the cleaned exhaust gas.